Because of variations in manufacturing process, both P-type and N-type Metal Oxide Semiconductor transistors (PMOS and NMOS transistors) in different wafers have different driving capabilities or different operational currents from the drain to the source (i.e., different currents IDSsat). A transistor is said to be “fast” (“F”) when the transistor while in operation has a high conduction capability and its current IDSsat is relatively high compared to a normal current IDSsat in a normal or average transistor (“T”). In contrast, the transistor is “slow” (“S”) when the transistor has a low conduction capability and current IDSsat of the transistor is relative low compared to a current IDSsat in a normal transistor. When a transistor is fast, the threshold voltage, e.g., voltage VT, of that transistor, is lower, and the transistor is turned on easier with a lower voltage applied at its gate. The symbols TT, FF, FS, SF, SS refer to a pair of an NMOS transistor and a PMOS transistor having the combinations of being normal-normal, fast-fast, fast-slow, slow-fast, or slow-slow, respectively.
Generally, standby and leakage current in Random Access Memories (SRAMs) vary depending on the driving capabilities of the transistors used in the SRAMs. In an approach related to SRAMs, an NMOS transistor formed as a diode and functioning as a footer is used to reduce the standby current caused by the leakage current. This approach, however, also increases the minimum operational voltage of the memory cell in the standby mode.
Like reference symbols in the various drawings indicate like elements.